Mammalian Ras proteins act as molecular switches in the signalling events associated with cell growth and differentiation. The ras proto-oncogene family consists of three members, N-, K-, and H-ras, which code for highly homologous 4 types of proteins; i.e., H, N-ras proteins of 189 residues and two isomorphic K-ras-4B and K-ras-4A proteins of 188 and 189 residues, respectively. The chemical basis for the switch mechanism involves cycling of the protein between the inactive (off) guanosine diphosphate (GDP) bound state and the active (on) guanosine triphosphate (GTP) bound state (Boune, H. R.; Sanders, D. A.; McCormick. F.; Nature, 1991, 349, 117). Biochemical and structural studies have shown that point mutations of the residues 12, 13 and 61, positioned in the neighborhood of phosphoryl ground of GTP, resulting in the decrease of guanosine triphosphatase activity are associated with many human cancers, particularly, pancreatic cancer, urinary bladder carcinoma, colon cancer, etc. (Bos, J. L., Cancer Res., 1989, 49, 4682).
Ras protein is synthesized as a cytosolic precursor that ultimately localized to the cytoplasmic face of the plasma membrane after a series of posttranslational modification (Gibbs, J. B., Cell 1991, 65, 1). These series of biochemical modifications, by changing the electrical charge state or spacial structure to increase the hydrophobicity allow Ras protein to attach to cell membrane more easily. The first and obligatory step in the series is the addition of a farnesyl moiety to the cysteine residue of the C-terminal CAAX motif (C, cysteine; A, usually aliphatic residue; X, any other amino acid) in a reaction catalyzed by farnesyl protein transferase (FTase). This modification is essential for Ras function, as demonstrated by the inability of Ras mutants lacking the C-terminal cysteine to be farnesylated, to localize to the plasma, and to transform mammalian cells in culture (Hancock, J. F., Magee, A. I., Childs, J. E., Marshall, C. J., Cell 1989, 57, 1167). The subsequent posttranslational modifications, cleavage of the AAX residues, carboxyl methylation of the the farnesylated cysteine, and palmitoylation of the cysteines located upstream of the CAAX motif in H- and N-ras proteins are not obligatory for Ras membrane association or cellular transforming activity. Interestingly, K-ras-4B, different from H- and N-ras, has a multiple lysine rich region named polybasic domain, instead of having cysteine required for palmitoylation, thereby facilitating the farnesylated ras protein to bind to anionic lipid layer of cell membrane. The inhibitors of FTase that catalyzes the obligatory modification have therefore been suggested as anticancer agents for tumors in which Ras oncogene contributes to transformation (Buses, J. E. et al., Chemistry & Biology, 1995, 2, 787). A number of FTase inhibitors recently identified demonstrated potent and specific ability to block Ras farnesylation, signalling and transformation in transformed cells and tumor cell lines both in vitro and in animal models (Kohl. N. E. et. al., Proc. Natl. Acad. Sci. USA. 1994, 91, 9141; Kohl, N. E. et al., Nature Medicine, 1995, 1 792).
However, most of the inhibitors are related to CAAX motif as Ras substrate mimic and peptidic in nature or contain a sulfhydryl group (U.S. Pat. No. 5,141,851; Kohl, N. E. et. al., Science, 1993, 260, 1934; PCT/US95/12224, Graham et al.; Sebti, S. M. et. al., J. Biol. Chem., 1995. 270, 26802; James, G. L. et al., Science, 1993, 260, 1937; Bishop, W. R. et al., J. Biol. Chem., 1995, 270, 30611). Recently, a new type of peptidomimetic inhibitor imitating catalytic step of FTase has been reported (Poulter, C. D. et al., J. Am. Chem. Soc., 1996, 118, 8761). The chemical basis of the inhibitor design relates to the reaction mechanism. This is, transferring prenyl group by the enzyme is electrophilic displacement and the reaction requires (+) charge in a transition state.
These inhibitors previously described, however, possess limited activity and selectivity for inhibition of the oncogenic function of Ras proteins, particularly K-ras-4B, which is found to be most common in human cancer. Therefore, new inhibitor having the ability of effectively inhibiting K-ras activity is required.
With regard to the restenosis and vascular proliferative diseases, it has been shown that inhibition of cellular ras prevents smooth muscle proliferation after vascular injury in vivo (Indolfi C. et al., Nature Med., 1995, 1(6), 541-545). This report definitively supports a role for farnesyl transferase inhibitors in this disease, showing inhibition of accumulation and proliferation of vascular smooth muscle.